Impairments in Adipose Tissue Microcirculation in Type 2 Diabetes Mellitus Assessed by Real-Time Contrast-Enhanced Ultrasound

Empagliflozin improves vascular insulin sensitivity and muscle perfusion in persons with type 2 diabetes

Sodium glucose cotransporter 2 inhibitors (SGLT2is) improved major adverse cardiovascular events (MACE), heart failure, and renal outcomes in large trials; however, a thorough understanding of the vascular physiological changes contributing to these responses is lacking. We hypothesized that SGLT2i therapy would diminish vascular insulin resistance and improve hemodynamic function, which could improve clinical outcomes. To test this, we treated 11 persons with type 2 diabetes for 12 wk with 10 mg/day empagliflozin and measured vascular stiffness, endothelial function, peripheral and central arterial pressures, skeletal and cardiac muscle perfusion, and vascular biomarkers before and at 120 min of a euglycemic hyperinsulinemic clamp at weeks 0 and 12. We found that before empagliflozin treatment, insulin infusion lowered peripheral and central aortic systolic pressure (P < 0.05) and muscle microvascular blood flow (P < 0.01), but showed no effect on other vascular measures. Following empagliflozin, insulin infusion improved endothelial function (P = 0.02), lowered peripheral and aortic systolic (each P < 0.01), diastolic (each P < 0.05), mean arterial (each P < 0.01), and pulse pressures (each P < 0.02), altered endothelial biomarker expression, and decreased radial artery forward and backward pressure amplitude (each P = 0.02). Empagliflozin also improved insulin-mediated skeletal and cardiac muscle microvascular perfusion (each P < 0.05). We conclude that empagliflozin enhances insulin's vascular actions, which could contribute to the improved cardiorenal outcomes seen with SGLT2i therapy.NEW & NOTEWORTHY The physiological underpinnings of the cardiovascular benefits of SGLT2 inhibitors remain uncertain. We tested whether empagliflozin mitigates vascular insulin resistance in patients with type 2 diabetes. Aortic and peripheral systolic, diastolic, mean and pulse pressures, endothelial function, vascular stiffness, and heart and muscle microvascular perfusion were measured before and during an insulin infusion at baseline and after 12 wk of empagliflozin. After empagliflozin, vascular responses to insulin improved dramatically.

Clinical and Translational Imaging and Sensing of Diabetic Microangiopathy: A Narrative Review

Microvascular changes in diabetes affect the function of several critical organs, such as the kidneys, heart, brain, eye, and skin, among others. The possibility of detecting such changes early enough in order to take appropriate actions renders the development of appropriate tools and techniques an imperative need. To this end, several sensing and imaging techniques have been developed or employed in the assessment of microangiopathy in patients with diabetes. Herein, we present such techniques; we provide insights into their principles of operation while discussing the characteristics that make them appropriate for such use. Finally, apart from already established techniques, we present novel ones with great translational potential, such as optoacoustic technologies, which are expected to enter clinical practice in the foreseeable future.

Hemodynamics and Arterial Stiffness in Response to Oral Glucose Loading in Individuals with Type II Diabetes and Controlled Hypertension

INTRODUCTION

Type 2 diabetes (T2D), the fastest growing pandemic, is typically accompanied by vascular complications. A central hallmark of both T2D and vascular disease is insulin resistance which causes impaired glucose transport and vasoconstriction concomitantly. Those with cardiometabolic disease display greater variation in central hemodynamics and arterial elasticity, both potent predictors of cardiovascular morbidity and mortality, which may be exacerbated by concomitant hyperglycemia and hyperinsulinemia during glucose testing. Thus, elucidating central and arterial responses to glucose testing in those with T2D may identify acute vascular pathophysiologies triggered by oral glucose loading.

AIM

This study compared hemodynamics and arterial stiffness to an oral glucose challenge (OGC: 50g glucose) between individuals with and without T2D. 21 healthy (48 ± 10 years) and 20 participants with clinically diagnosed T2D and controlled hypertension (52 ± 8 years) were tested.

METHODS

Hemodynamics and arterial compliance were assessed at baseline, and 10, 20, 30, 40, 50, and 60 min post-OGC.

RESULTS

Heart rate increased between 20 and 60 post-OGC in both groups (p < 0.05). Central systolic blood pressure (SBP) decreased in the T2D group between 10 and 50 min post-OGC while central diastolic blood pressure (DBP) decreased in both groups from 20 to 60 post-OGC. Central SBP decreased in T2D between 10 and 50 min post-OGC and central DBP decreased in both groups between 20 and 60 min post-OGC. Brachial SBP decreased between 10 and 50 min in healthy participants, whereas both groups displayed decreases in brachial DBP between 20 and 60 min post-OGC. Arterial stiffness was unaffected.

CONCLUSIONS

An OGC alters central and peripheral blood pressure in healthy and T2D participants similarly with no changes in arterial stiffness.

Oral and intravenous glucose administration elicit opposing microvascular blood flow responses in skeletal muscle of healthy people: role of incretins

Abstract

Insulin infusion increases skeletal muscle microvascular blood flow (MBF) in healthy people but is impaired during insulin resistance. However, we have shown that eliciting insulin secretion via oral glucose loading in healthy people impairs muscle MBF, whilst others have demonstrated intravenous glucose infusion stimulates MBF. We aimed to show that the route of glucose administration (oral versus intravenous) influences muscle MBF, and explore potential gut‐derived hormones that may explain these divergent responses. Ten healthy individuals underwent a 120 min oral glucose tolerance test (OGTT; 75 g glucose) and on a subsequent occasion an intravenous glucose tolerance test (IVGTT, bypassing the gut) matched for similar blood glucose excursions. Femoral artery and thigh muscle microvascular (contrast‐enhanced ultrasound) haemodynamics were measured at baseline and during the OGTT/IVGTT. Plasma insulin, C‐peptide, glucagon, non‐esterified fatty acids and a range of gut‐derived hormones and incretins (gastric inhibitory polypeptide (GIP) and glucagon‐like peptide‐1(GLP‐1)) were measured at baseline and throughout the OGTT/IVGTT. The IVGTT increased whereas the OGTT impaired MBF (1.3‐fold versus 0.5‐fold from baseline, respectively, P = 0.0006). The impairment in MBF during the OGTT occurred despite producing 2.8‐fold higher plasma insulin concentrations (P = 0.0001). The change in MBF from baseline (ΔMBF) negatively correlated with ΔGIP concentrations (r = −0.665, P < 0.0001). The natural log ratio of incretins GLP‐1:GIP was positively associated with ΔMBF (r = 0.658, P < 0.0001), suggesting they have opposing actions on the microvasculature. Postprandial hyperglycaemia per se does not acutely determine opposing microvascular responses between OGTT and IVGTT. Incretins may play a role in modulating skeletal muscle MBF in humans.

Key points

Insulin or mixed nutrient meals stimulate skeletal muscle microvascular blood flow (MBF) to aid in the delivery of nutrients; however, this vascular effect is lost during insulin resistance.

Food/drinks containing large glucose loads impair MBF in healthy people; however, this impairment is not observed when glucose is infused intravenously (bypassing the gut).

We investigated skeletal muscle MBF responses to a 75 g oral glucose tolerance test and intravenous glucose infusion and aimed to identify potential gut hormones responsible for glucose‐mediated changes in MBF.

Despite similar blood glucose concentrations, orally ingested glucose impaired, whereas intravenously infused glucose augmented, skeletal muscle MBF. The incretin gastric inhibitory polypeptide was negatively associated with MBF, suggestive of an incretin‐mediated MBF response to oral glucose ingestion.

This work provides new insight into why diets high in glucose may be detrimental to vascular health and provides new avenues for novel treatment strategies targeting microvascular dysfunction.

Aerobic Exercise Training Improves Microvascular Function and Oxidative Stress Parameters in Diet-Induced Type 2 Diabetic Mice

PURPOSE

Type 2 diabetic (T2D) patients have liver and adipose tissue microcirculation disturbances associated with metabolic dysfunction and disease progression. However, the potential role of aerobic training on hepatic and white adipose tissue (WAT) microcirculation and the underlying mechanisms have not been elucidated to date. Therefore, we investigated the role of aerobic training on liver and WAT microcirculation and AGE-RAGE modulation in T2D mice.

METHODS

The control group (CTL) was fed standard chow, and T2D was induced by feeding male C57BL/6 a high-fat, high-carbohydrate diet for 24 weeks. In the following 12 weeks, mice underwent aerobic training (CTL EX and T2D EX groups), or were kept sedentary (CTL and T2D groups). We assessed metabolic parameters, biochemical markers, oxidative damage, the AGE-RAGE axis, hepatic steatosis, hepatic stellate cells activation (HSC) and liver and WAT microcirculation.

RESULTS

Hepatic microcirculation was improved in T2D EX mice which were associated with improvements in body, liver and fat mass, blood pressure, hepatic steatosis and fibrosis, and decreased HSC and AGE-RAGE activation. In contrast, improvement in WAT microcirculation, that is, decreased leukocyte recruitment and increased perfusion, was associated with increased catalase antioxidant activity.

CONCLUSION

Physical training improves hepatic and adipose tissue microcirculatory dysfunction associated with T2D, likely due to downregulation of AGE-RAGE axis, decreased HSC activation and increased antioxidant activity.

Measurement of human abdominal and femoral intravascular adipose tissue blood flow using percutaneous Doppler ultrasound

Adipose tissue blood flow (ATBF) is an important determinant of adipose tissue (AT) function. ¹³³Xenon wash-out technique is considered the gold-standard for human ATBF measurements. However, decreasing ¹³³Xenon clinical use and costly production and preservation, make alternative (non-invasive) methods necessary. Here, we explored percutaneous Doppler ultrasound as a proxy method to quantify intravascular subcutaneous abdominal and femoral ATBF in humans (n= 17). Both fasting ATBF and the postprandial increase in ATBF were significantly higher in abdominal compared to femoral AT. Although anatomical variations in vein location and depot thickness may impact feasibility, we demonstrate that Doppler ultrasound detects the expected depot-differences and postprandial increase in ATBF in healthy individuals. This method warrants further investigation in other populations and metabolic conditions.

Diabetes pathogenesis and management: the endothelium comes of age

Endothelium, acting as a barrier, protects tissues against factors that provoke insulin resistance and type 2 diabetes and itself responds to the insult of insulin resistance inducers with altered function. Endothelial insulin resistance and vascular dysfunction occur early in the evolution of insulin resistance-related disease, can co-exist with and even contribute to the development of metabolic insulin resistance, and promote vascular complications in those affected. The impact of endothelial insulin resistance and vascular dysfunction varies depending on the blood vessel size and location, resulting in decreased arterial plasticity, increased atherosclerosis and vascular resistance, and decreased tissue perfusion. Women with insulin resistance and diabetes are disproportionately impacted by cardiovascular disease, likely related to differential sex-hormone endothelium effects. Thus, reducing endothelial insulin resistance and improving endothelial function in the conduit arteries may reduce atherosclerotic complications, in the resistance arteries lead to better blood pressure control, and in the microvasculature lead to less microvascular complications and more effective tissue perfusion. Multiple diabetes therapeutic modalities, including medications and exercise training, improve endothelial insulin action and vascular function. This action may delay the onset of type 2 diabetes and/or its complications, making the vascular endothelium an attractive therapeutic target for type 2 diabetes and potentially type 1 diabetes.

Supplemental Watermelon Juice Attenuates Acute Hyperglycemia-Induced Macro-and Microvascular Dysfunction in Healthy Adults

BACKGROUND

Acute hyperglycemia reduces NO bioavailability and causes macro- and microvascular dysfunction. Watermelon juice (WMJ) is a natural source of the amino acid citrulline, which is metabolized to form arginine for the NO cycle and may improve vascular function.

OBJECTIVES

We examined the effects of 2 weeks of WMJ compared to a calorie-matched placebo (PLA) to attenuate acute hyperglycemia-induced vascular dysfunction.

METHODS

In a randomized, placebo-controlled, double-blind, crossover trial, 6 men and 11 women (aged 21-25; BMI, 23.5 ± 3.2 kg/m2) received 2 weeks of daily WMJ (500 mL) or a PLA drink followed by an oral-glucose-tolerance test. Postprandial flow-mediated dilation (FMD) was measured by ultrasound (primary outcome), while postprandial microvascular blood flow (MVBF) and ischemic reperfusion were measured by near-infrared spectroscopy (NIRS) vascular occlusion test (VOT).

RESULTS

The postprandial FMD area AUC was higher after WMJ supplementation compared to PLA supplementation (838 ± 459% · 90 min compared with 539 ± 278% · 90 min; P = 0.03). The postprandial MVBF (AUC) was higher (P = 0.01) following WMJ supplementation (51.0 ± 29.1 mL blood · 100 mL tissue-1 · min-1 · 90 min) compared to the PLA (36.0 ± 20.5 mL blood · 100 mL tissue-1 · min-1 · 90 min; P = 0.01). There was a significant treatment effect (P = 0.048) for WMJ supplementation (71.2 ± 1.5%) to increase baseline tissue oxygen saturation (StO2%) when compared to PLA (65.9 ± 1.7%). The ischemic-reperfusion slope was not affected by WMJ treatment (P = 0.83).

CONCLUSIONS

Two weeks of daily WMJ supplementation improved FMD and some aspects of microvascular function (NIRS-VOT) during experimentally induced acute hyperglycemia in healthy adults. Preserved postprandial endothelial function and enhanced skeletal muscle StO2% are likely partially mediated by increased NO production (via citrulline conversion into arginine) and by the potential antioxidant effect of other bioactive compounds in WMJ.

High-glucose mixed-nutrient meal ingestion impairs skeletal muscle microvascular blood flow in healthy young men

Oral glucose ingestion leads to impaired muscle microvascular blood flow (MBF), which may contribute to acute hyperglycemia-induced insulin resistance. We investigated whether incorporating lipids and protein into a high-glucose load would prevent postprandial MBF dysfunction. Ten healthy young men (age, 27 yr [24, 30], mean with lower and upper bounds of the 95% confidence interval; height, 180 cm [174, 185]; weight, 77 kg [70, 84]) ingested a high-glucose (1.1 g/kg glucose) mixed-nutrient meal (10 kcal/kg; 45% carbohydrate, 20% protein, and 35% fat) in the morning after an overnight fast. Femoral arterial blood flow was measured via Doppler ultrasound, and thigh MBF was measured via contrast-enhanced ultrasound, before meal ingestion and 1 h and 2 h postprandially. Blood glucose and plasma insulin were measured at baseline and every 15 min throughout the 2-h postprandial period. Compared with baseline, thigh muscle microvascular blood volume, velocity, and flow were significantly impaired at 60 min postprandial (-25%, -27%, and -46%, respectively; all P < 0.05) and to a greater extent at 120 min postprandial (-37%, -46%, and -64%; all P < 0.01). Heart rate and femoral arterial diameter, blood velocity, and blood flow were significantly increased at 60 min and 120 min postprandial (all P < 0.05). Higher blood glucose area under the curve was correlated with greater MBF dysfunction (R² = 0.742; P < 0.001). Ingestion of a high-glucose mixed-nutrient meal impairs MBF in healthy individuals for up to 2 h postprandial.

Transcranial contrast-enhanced ultrasound in the rat brain reveals substantial hyperperfusion acutely post-stroke

Direct and real-time assessment of cerebral hemodynamics is key to improving our understanding of cerebral blood flow regulation in health and disease states such as stroke. While a number of sophisticated imaging platforms enable assessment of cerebral perfusion, most are limited either spatially or temporally. Here, we applied transcranial contrast-enhanced ultrasound (CEU) to measure cerebral perfusion in real-time through the intact rat skull before, during and after ischemic stroke, induced by intraluminal filament middle cerebral artery occlusion (MCAO). We demonstrate expected decreases in cortical and striatal blood volume, flow velocity and perfusion during MCAO. After filament retraction, blood volume and perfusion increased two-fold above baseline, indicative of acute hyperperfusion. Adjacent brain regions to the ischemic area and the contralateral hemisphere had increased blood volume during MCAO. We assessed our data using wavelet analysis to demonstrate striking vasomotion changes in the ischemic and contralateral cortices during MCAO and reperfusion. In conclusion, we demonstrate the application of CEU for real-time assessment of cerebral hemodynamics and show that the ischemic regions exhibit striking hyperemia post-MCAO. Whether this post-stoke hyperperfusion is sustained long-term and contributes to stroke severity is not known.

Oxygen dependence of glucose sensing: role in glucose homeostasis and related pathology

In glucose homeostasis, glucose concentration is sensed by its metabolism through glucokinase (GCK) and oxidative phosphorylation. Because oxidative phosphorylation is an integral part of the sensory system, glucose sensing is necessarily dependent on oxygen pressure. Much of the dependence on oxygen is suppressed by location of glucose sensing cells in tissues with well-regulated blood flow. In healthy individuals the oxygen dependence is primarily observed in response to transient global hypoxia events such as during birth or transition to high altitude. The GCK sensing system is, however, used to control release of both insulin and glucagon, the preeminant hormonal regulators of blood glucose, as well as glucose sensitive neuronal activity. Suppression of oxygen delivery to glucose-sensing cells or interference with regulation of tissue blood flow by either local or systemic causes, stresses the glucose regulatory system. This is true whether the stress is imposed locally, such as by altered oxygen delivery to the pancreas, or globally, as in pulmonary insufficiency or exposure to high altitude. It may be expected that chronic application of this stress predisposes individuals to developing diabetes. Type 2 diabetes is a broad class of diseases characterized by disturbance of glucose homeostasis, i.e., having either hyperglycemia and/or decreased sensitivity to insulin. Given the role of oxidative phosphorylation in glucose sensing, tissue oxygen deprivation may predispose individuals to developing diabetes as well as contributing to the disease itself. This is particularly true in age-related diabetes because the incidence of vascular insufficiency increases markedly with increasing age. NEW & NOTEWORTHY Glucose sensing requires glucose metabolism through glycolysis and oxidative phosphorylation. Dependence of the latter on oxygen concentration imposes an oxygen dependence on glucose sensing. We have used a validated computational model to quantify that dependence. Evidence is presented that tissue oxygenation plays an important role in predisposition of individuals to developing type 2 diabetes and in progression of the disease.

Are the metabolic benefits of resistance training in type 2 diabetes linked to improvements in adipose tissue microvascular blood flow?

The microcirculation in adipose tissue is markedly impaired in type 2 diabetes (T2D). Resistance training (RT) often increases muscle mass and promotes a favorable metabolic profile in people with T2D, even in the absence of fat loss. Whether the metabolic benefits of RT in T2D are linked to improvements in adipose tissue microvascular blood flow is unknown. Eighteen sedentary people with T2D (7 women/11 men, 52 ± 7 yr) completed 6 wk of RT. Before and after RT, overnight-fasted participants had blood sampled for clinical chemistries (glucose, insulin, lipids, HbA1c, and proinflammatory markers) and underwent an oral glucose challenge (OGC; 50 g glucose × 2 h) and a DEXA scan to assess body composition. Adipose tissue microvascular blood volume and flow were assessed at rest and 1 h post-OGC using contrast-enhanced ultrasound. RT significantly reduced fasting blood glucose ( P = 0.006), HbA1c ( P = 0.007), 2-h glucose area under the time curve post-OGC ( P = 0.014), and homeostatic model assessment of insulin resistance ( P = 0.005). This was accompanied by a small reduction in total body fat ( P = 0.002), trunk fat ( P = 0.023), and fasting triglyceride levels ( P = 0.029). Lean mass ( P = 0.003), circulating TNF-α ( P = 0.006), and soluble VCAM-1 ( P < 0.001) increased post-RT. There were no significant changes in adipose tissue microvascular blood volume or flow following RT; however those who did have a higher baseline microvascular blood flow post-RT also had lower fasting triglyceride levels ( r = -0.476, P = 0.045). The anthropometric, glycemic, and insulin-sensitizing benefits of 6 wk of RT in people with T2D are not associated with an improvement in adipose tissue microvascular responses; however, there may be an adipose tissue microvascular-linked benefit to fasting triglyceride levels.